1. Field of the Invention
The present invention relates to an electrochemical process for the selective preparation of partially oxidized organic compounds.
2. Discussion of the Background
The direct selective oxidation of organic compounds has hitherto been possible only in a few cases, since the partially oxidized products are usually more reactive than the starting materials used, which leads to complete oxidation to form carbon dioxide. In particular, the problem of the direct oxidation of alkanes has up to now not been able to be solved satisfactorily.
Only maleic anhydride can be produced by direct oxidation using n-butane as starting material. In the above process, the stabilization of the oxidation product by ring formation plays a decisive role.
Many attempts to carry out the partial direct oxidation of nonreactive organic compounds have concentrated on the development of new heterogeneous catalysts, but the yield of the partially oxidized product is frequently not industrially satisfactory.
In comparison, less attention has been paid to electrochemical partial oxidation. In this area, the main focus of development work was, in contrast, the utilization of total oxidation of suitable compounds for the generation of electric energy in fuel cells.
An example of the electrochemical oxidation of organic compounds is described in U.S. Pat. No. 4,329,208, namely the oxidation of ethene to give ethylene oxide. This oxidation is carried out at an anode consisting of silver or a silver alloy by means of a solid electrolyte system comprising zirconium oxides.
Another process for the electrochemical oxidation of organic compounds is disclosed in U.S. Pat. No. 4,661,422. Here, hydrocarbons are oxidized at a metal/metal oxide anode in a salt melt as electrolyte. The salt melt comprises carbonate, nitrate or sulfate salts while the cathode is made up of mixed oxides of metals of groups I B, II B, III A, V B, VI B, VII B and VIII of the Periodic Table.
In Catalysis Today, 25, 371 (1995), Takehira et al, studied the partial oxidation of propene in a construction similar to a fuel cell. As electrolyte, they used Y-stabilized ZrO.sub.2. The anode material employed was Au coated with an Mo-Bi mixed oxide as catalysts and the cathode material was Ag. The reaction temperature was 475.degree. C.
The yield of the oxidation product desired in each case is generally so low that none of these processes has industrial relevance. Here, too, the problem of the total oxidation of the organic substrate to carbon dioxide has not yet been solved. In addition, the electrolyte acts as an "oxygen pump", i.e. the oxygen required for the oxidation is reduced at the cathode and then migrates in ionic form through the electrolyte to the anode. The anode space contains only the substrate to be oxidized and possibly an inert gas. The feeding of oxygen into the anode space does not lead to an increase in the yield of the desired oxidation product.
Another disadvantage is that the reaction temperature is determined by the oxygen conductivity of the electrolyte. The electrolytes used have a sufficient conductivity only at temperatures which are significantly above the optimum temperatures for such oxidation reactions, which must partly explain the low selectivity of the processes examined. Particularly, processes which use salt melts as electrolyte are forced to have such high reaction temperatures (up to 750.degree. C.) that decomposition of the product is virtually unavoidable. Processes of this type are unsuitable for preparing thermally unstable compounds (e.g., Michael systems). The discovery of the NEMCA effect (Non Faradaic Electrochemical Modification of Catalytic Activity) opened up the opportunity of developing more economical electrochemical processes. In "Studies in Surface Science and Catalysis", R. K. Grasselli, S. T. Oyama, A. M. Gaffney, J. E. Lyons (Editors), 110, 77 (1997) and Science 264, 1563 (1994), Vayenas et al described an electrochemical process based on a conductive, porous metal (oxide) film on a solid electrolyte such as Y-stabilized ZrO.sub.2. Gastight separation of the anode and cathode spaces is no longer necessary here and the oxidizing agent can also be fed into the anode space. However, it was found that the main product of the oxidation, carbon dioxide, still results from the total oxidation of the substrate and the selectivity to a desired partially oxidized product is very low even at low conversions.